POSIX: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/uniq.html
Ajacent dupe line operations.
Remove adjacent dupes lines:
[ "$(printf 'a\nb\n' | uniq )" = "$(printf "a\nb")" ] || exit 1
[ "$(printf 'a\na\n' | uniq )" = 'a' ] || exit 1
Non-adjacent dupes are not removed:
[ "$(printf 'a\nb\na\na\n' | uniq )" = "$( printf "a\nb\na")" ] || exit 1
Thus the sort combo:
[ "$(printf 'a\nb\na\na\n' | sort | uniq )" = "$(printf "a\nb")" ] || exit 1
Other options:
-u: only show lines that have no dupe-d: dupe lines only-c: shows dupe count before each line